1. Field of the Invention
The present invention relates to electroluminescent (hereinafter referred to as EL) devices which are used in various instruments of, for example, emissive-type segment displays and matrix displays, and in displays in various information terminals. The present invention also relates to methods for producing the same.
2. Related Arts
An EL device is typically formed by laminating a first electrode, a first insulating layer, an luminescent layer, a second insulating layer, and a second electrode on an insulating glass substrate in this order, and the layers at least on one side of the luminescent layer are made of transparent materials.
The luminescent layer is composed of a host material doped with a luminescent center, and various luminous materials are previously proposed. Especially, a luminescent layer capable of emitting blue light, including an alkaline earth thiogallate (MGa2S4, M=Ca, Sr, Ba) as the host material doped with cerium (Ce) as the luminescent center, is known. The alkaline earth thiogallate luminescent layer is deposited by a sputtering method.
However, in the case that the alkaline earth thiogallate layer formed by the sputtering method is independently used as the luminescent layer in the EL device, there occurs a problem that the luminescent layer is liable to peel off during thermal annealing after the luminescent layer is deposited. The delamination results in a decline in the breakdown voltage of the EL device, and the breakdown of the EL device starts at some tens volts over a threshold voltage. Therefore, reliability of the EL device is lowered.
As opposed to this, an EL device including an alkaline earth thiogallate luminescent layer deposited by sputtering and interposed between a pair of zinc sulfide (ZnS) layers to prevent delamination is disclosed in JP-A-5-65478.
The ZnS layers, however, do not contribute to the luminescent process, and the threshold voltage of the EL devices is increased by the ZnS layers.